Magnetoelectronic devices, spin electronic devices, and spintronic devices are synonymous terms for devices that make use of effects predominantly caused by electron spin. Magnetoelectronics are used in numerous information devices to provide non-volatile, reliable, radiation resistant, and high-density data storage and retrieval. The numerous magnetoelectronics information devices include, but are not limited to, Magnetoresistive Random Access Memory (MRAM), magnetic sensors, and read/write heads for disk drives.
Typically an MRAM includes an array of magnetoresistive memory elements. Each magnetoresistive memory element typically has a structure that includes multiple magnetic layers separated by various non-magnetic layers, such as a magnetic tunnel junction (MTJ), and exhibits an electrical resistance that depends on the magnetic state of the device. Information is stored as directions of magnetization vectors in the magnetic layers. Magnetization vectors in one magnetic layer are magnetically fixed or pinned, while the magnetization direction of another magnetic layer may be free to switch between the same and opposite directions that are called “parallel” and “antiparallel” states, respectively. Corresponding to the parallel and antiparallel magnetic states, the magnetic memory element has low and high electrical resistance states, respectively. Accordingly, a detection of the resistance allows a magnetoresistive memory element, such as an MTJ device, to provide information stored in the magnetic memory element. There are two completely different methods used to program the free layer: field-switching and spin-torque switching. In field-switched MRAM, current carrying lines adjacent to the MTJ bit are used to generate magnetic fields that act on the free layer. In spin-torque MRAM, switching is accomplished with a current pulse through the MTJ itself. The spin angular momentum carried by the spin-polarized tunneling current causes reversal of the free layer, with the final state (parallel or antiparallel) determined by the polarity of the current pulse. Spin-torque transfer is known to occur in MTJ devices and giant magnetoresistance devices that are patterned or otherwise arranged so that the current flows substantially perpendicular to the interfaces, and in simple wire-like structures when the current flows substantially perpendicular to a domain wall. Any such structure that exhibits magnetoresistance has the potential to be a spin-torque magnetoresistive memory element. The mean current required to switch the magnetic state of the free layer is called the critical current (Ic). The critical current density (Jc) is the average critical current per area of the bit (Jc=Ic/A), where A is the area, and the current supplied by the circuit to switch spin-torque MRAM elements in a memory array is the write current (Iw). Reducing Iw is desirable so that a smaller access transistor can be used for each bit cell and a higher density, lower cost memory can be produced. Lowering Jc while not lowering the break down voltage, Vbd, where the tunnel barrier breaks down is desirable. Tunnel barrier breakdown is an irreversible degradation in the integrity of the tunnel barrier so that the magnetoresistance and spin torque reliability are greatly reduced.
Reducing the Ic by lowering the area of the bit for a constant Jc reduces the magnetic energy barrier Eb separating the two stable states of the free layer. Eb is proportional to the magnetization Ms of the free layer material, the anisotropy Hk of free layer, and the free layer volume V. Reducing the area obviously reduces V and therefore Eb. Reducing Eb impacts the non-volatility of the MRAM making it possible for the free layer to switch via thermal fluctuations in the temperature range and during the operating life of the part. It would be therefore be advantageous to reduce the area of the tunnel barrier without the reduction in area of the free layer.
In order to reduce write current, some spin-torque MRAM elements incorporate a dual-spin-filter (DSF) structure, in which the MTJ stack includes two different spin-polarizing layers, one on each side of the free layer, to lower Jc by improving spin-torque transfer efficiency through increased spin torque on the free layer, resulting in a lower write current. Dual-spin-filter devices have two tunnel barriers for providing a lower Jc, and a more symmetrical write current in the current up/down direction, than found in single tunnel barrier devices.
Dual-spin-filter devices require that the spin-polarizing fixed layers on either side of the free layer have opposite magnetization directions, so that the spin-torque effect from each of the two fixed layers will act together to switch the free layer magnetization into the desired direction when a current flows either up or down through the device.
Another structure similar to the DSF structure is a dual tunnel barrier structure (DTB). This structure, like the DSF, has tunnel barriers on either side of the free layer but, unlike the DSF, a magnetic fixed layer on only one side. This structure has shown an improvement in the ratio of the voltage required to write the bits and the voltage where breakdown occurs. This ratio provides more operating margin or voltage (or current Iw) and therefore is advantageous.
Accordingly, it is desirable to provide a spin-torque magnetoresistive memory element having symmetric tunnel barriers resulting in improved symmetry in the polarized current and switching states, the upper tunnel barrier undamaged by etching through the free layer. Furthermore, other desirable features and characteristics of the exemplary embodiments will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background.